Circuit for detection of frequency differences and apparatus employing same



Dec. 30, 1952 s. M. BAGNO 2,623,931

- CIRCUIT FOR DETEC ION OF FREQUENCY DIFFERENCES AND APPARATUS EMPLOYING SAME Original `Filed Sept. 26, 1947 '3 Sheets-Sheel l INTEGRATOQHELAY c' eEFL'E cTorzlimlcom-lonfi-I AMPuFsEH DETEcnH :zum x (34 J6 mE-r Fl HAMPuFIEQ HmicTvFlEg-l -LAMPLIFizHar-:c

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I IN V EN TOR. .5H/w54 M. .ena/vo ZLMMM A #from/521g.

TRANSMIIT' De 30, 1952 s. M. BAGNO i CIRCUIT FOR DETECTION OF FREQUENCY DIFFERENCES AND APPARATUS EMPLOYING SAME Original Filed Sept. 26, 1947 -3 Sheets-Sheet 2 NNI l-ll- INVENToR. .MM/f2 M. 5pm/ o Dec. ,30, 1952 s. M. BAGNO 2,623,931

CIRCUIT FOR DETECTIGN 0F FREQUENCY DIFFERENCES AND APPARATUS EMPLOYING SAME Original Filed Sept. 26, 1947 3 Sheets-Sheet 3 INVENTOR.

w M M m w 5A m N m n A Patented Dec. 30, 1952 UNITED STAT-ES PATENT' OFFICE CIRCUIT FDR DETECTION OF FREQUENCY. DIFFERENCESL AND APPARATUS EMPLOY- INGLSAMEC.

Samuel.M.'.Bagno, Astoria, N.. Y., assignor to The Alertronic Protective Corporation of America, New York, N. YL, incorporation of NewYork Original :application September 26, 1947, Sierial': No. 776,368. Divided andv this applicationDecember. 4,. 1948,.,Serial No.. 63,585

21 Claims;

'Thepresentinvention relates .to anos/el circuitv` forthe detection of .frequency differences .and to the use `of such a circuit toactuate a. switch. or other control device depending upon the existence or nonexistence ofsuch a frequency difference. The use .of sucha circuit vis here .disclosed .in conjunction with. an .apparatus for. detecting motion in a .confined space as disclosed more fully in applicantfs copending application, Serial No. 776,368, filed September 26, v1947, andlentitled Method and Apparatus for Detecting Motion .in a Confined Space, of. which this application is a division. It .willbe understoodhowever,. that thesubject matterof this application` may be used in apparatus other than that disclosed .in theparent application.

.The method and apparatus of. the parent appli.-

cation constitutes a burglar and fire alarm `which is actuated byreason of .the fact that motion of an intruder in a .given space or the existence in sucha. space of a thermal disturbance, be it fire, anoverheated radiatore or the like,y whensuchv al space islled with vibrations of a predetermined. frequency or frequencies,` preferably of super-y sonic value, willgive-riseto-the existence, .in such a.space,-of eifectivefrequencies of .vibrationzdifi fering from the predetermined frequencies-andv normally differing from saidpredetermined fr.e.

quencies by an amount of a: different order-of magnitude from said predeterminedV frequencies.

Whenthose soundwibrations of different fre'- quenciesa. are converted into electrical Vibrations of.; corresponding frequencies, .and when. .those electrical'. vibrations are .passed through a .detect-A ing apparatus .capable of :distinguishing between. the` frequencies and Vof .actuating vsom-e :control mechanism,y such as, 1a, switch. which.v controls an alarm,..on,: the basis; ofthe presence orabsence of such differing frequencies., an;;operab1e..alarmzsyse tem isprogduced.

The circuit; and-control.apparatusv as'herein disev closed are particularly. adaptedfor'usein suchxxa., Byi properly electrically detecting. apparatus.. combining a `-plural-ity `.of rectifiers .and frequency discriminating.A networks, a circuit arrangement is produced which'gives rise to different `outputs depending upon thepresence or absence of frequencies of predetermined magnitudes. Also disclosed herein is a control circuit actuated by certain signals` from said frequency discriminating` circuit, saidcontrol circuit being so arranged as. to .actuate a `switch or othercontrol device-in accordance with the .signals producedbythe frequencydetecting circuit.

Itis the prime object of the present invention to devise a noveltype of frequency` detecting cir cuit particularlyV adapted to detect. and signalthe presence or absence `oftwo dissimilar frequencies-.'

It 'is another object of `the 'present 'invention to* combin'e thesaid frequencyfdetecting; circuitwithLC a control circuit Which'is capable of-actuati'ng'ar switch or other control device 'in accordance with the presence or'absence of saidtwo differentfre` quencies.

Yet Aanother 4objectx'oi` the `present l inventiontis:

to devise a'combination of frequency discriminar` ing Vcircuit an'd control vcircuit so vconstructedanci arranged .as tocause .an alarm or-other signal to be actuated whenever twoi spe'ciiiedfrequencyfor frequencies within two specified rangesare'prese ent and effective to keep 'said alarm or. other siga nal from beinghactuated when only onev of said;

frequencies or ranges of frequencies is' present.

A still further object of 'the present. invention.A is to devise a combination frequencyv discriminateing circuitand control circuitlwhic'h ispositive. in operation and .which willrshould any vpart .of.Y the circuit. fail, cause the .alarm vor. .other .signal to be actuated.

To Atheaccomplishment oftheforegoingggob-if jects and suchv other objectsvas ymay hereinafter.

appear, the .present .invention .irelatesto theare rangement of a frequency' discriminating-circuit and a control circuitassociated thereWith',.-.a1l :as

defined in the-appended claimsand asfdescribed in this specification, .takenz together. .with the-ac-s companyingfdrawings, in which:

Figi isa schematic.representationota con-1 iined.. space suchcas a .room Lwhich tis sequipped.;

with the apparatus. for `detecting .motion .as :deeV

scribed'inthe parent application, said' apparatus f includingthe frequency `detecting-circuit andconeV trol circuit of the present invention;

Fig. 2- is a-blocky diagram illustratingthebroad steps i-nherent in -onemethod -of motion det'ection .employed therein:

Fig. 3fis a block diagram vvillustrating the detailsof the detector step, said'stepincludingithe'freL quencyv detecting circuit and' control circuitiof' the present invention;

Fig'l is a circuit diagram' showing the details of circuit design furthe amplifier;detector;'fre-- quency detectingL circuit, control circuit and relay;

Fig. 5`is a circuit diagramv typical of .the trans.-

mitters employed in the present invention; and

Fig. 6 isa circuit diagram ofthe power sup-r ply connections for the velectrical 'apparatus of Since one of the most important applications of use of the present invention is in a burglar or fire alarm of the type disclosed in the parent application, the illustration of Fig. 1 shows the manner of employment of such a motion detecting apparatus. In a confined space such as a room A, some of the walls of which are broken away in Fig. 1 to show the interior thereof, one or more transmitters B are positioned so as to transmit vibrations designated by the curved lines C into the room A. The frequency of these vibrations is preferably chosen to be somewhat higher than the range of frequencies audible to the human ear so that the operation of the equipment will be unknown to the would-be intruder and so that it will not interfere with the sensibilities of passers-by or those working adjacent to the room in which the apparatus is functioning. The contents of the room, here illustrated as a desk 2 and a chair 4, and the walls thereof will reflect the vibrations C, these reflected vibrations being designated by the curved lines C'. For purposes of illustration, these'reflections are shown as coming only from the desk 2, but it will be understood that the entire room A is filled both with the transmitted vibrations C and the reflected vibrations C. A microphone D capable of receiving vibrations of the frequencies involved and converting those vibrations into corresponding electrical fluctuations is also mounted in the room, and is so positioned as to receive not only the reflected vibrations C but the transmitted vibrations C. Since reflection takes place from al1 of the walls of the room, and since therefore, reflected waves are travelling in all directions, these reflected waves eventually, after repeated reections and reflections reaching the microphone D, the direction of motion which the apparatus will detect is not critical. No matter in what direction the intruder moves or the air accelerates, it will change the effective frequency of some waves and thus will eventually control the alarm. If the room is undisturbed, that is to say, if the contents thereof remain stationary, if the air therein is not accelerated, and if nothing moves in the room, the frequency of the reflected waves C will be equal to the frequency of the transmitted waves C. On the other hand, if there be a disturbance in the room, the waves C which are reflected by the thing in motion will differ in frequency from the transmitted waves C insofar as the microphone D is concerned. Thus, if .the desk 2 be moved toward the microphone D, the waves reflected therefrom will impinge upon the microphone D at a faster rate than if the desk 2 were stationary, or if the desk 2 be moved away from the microphone D, the waves C reflected therefrom will impinge upon the microphone D at a slower rate. The difference in frequency between transmitted waves C and the reflected waves C will therefore depend upon the speed with which vthe desk 2 is moved. Should a person enter the room, he, too, would reect the transmitted waves C, and should he move, the waves C reflected by him would appear to the microphone D as having a frequency different from that of the transmitted waves C. This effect is well known and is termed the Doppler effect in the literature. Since the frequency generated by motion within the room is derived from the original transmitted frequency, it might be considered in the sense that the original transmitted frequency is modulated by the motion within the room and hence what the micro- 4 phone D receives is the original transmitted frequency and its modulation.

Since the transmitted waves C must pass through the air in the room A, the condition of the air must also be taken into account. If the air is in motion and accelerating, the transmitted waves C carried thereby will, when they impinge upon the microphone D, have an apparent frequency equal to the transmitted frequency as modified by the motion of the air. Because of the fact that the transmitted waves C will reach the microphone by diverse routes, some of the waves travelling directly thereto (see the vertical wave train of Fig. l) and other of the wave trains C reaching the microphone by a more diverse route (see the reflected wave train C of Fig. 1), an effective difference in frequency will be detected by the circuit to which the microphone is connected.

Thus, if the window 6 or the door 8 of the room A be opened and if a draft enter therein, the air in the room A will be accelerated and thus the microphone D will detect frequency differences. Moreover, if any portion of the room be subjected to undue thermal disturbances, the air in the vicinity of those thermal disturbances will have considerable turbulence, naturally accompanied by acceleration. The motion of that portion of the air will so affect the transmitted waves C passing therethrough that when those waves are reflected and nally reach the microphone D, the circuit to which the microphone is connected will detect a frequency difference.

It will therefore be apparent that the disclosed method for detecting motion will be effective not only in detecting any motion of an intruder within the room A but will also be effective to detect any excessive air acceleration such as might be caused by a broken window or by undue thermal disturbance such as re.

. A certain amount of thermal disturbance in a room, and consequently a certain amount of air turbulence connected therewith, must be allowed for. If the room has a radiator and the radiator is in operation, air turbulence and acceleration will be present and it is necessary that any workable alarm system be so designed as not to be set off by such normal turbulence, the alarm only being soundable when the thermal disturbance exceeds the normal amount.

I have found that occasionally the placing of objects in the room sets up such a standing wave pattern that the microphone D is located at a null point. This furnishes the basis for detection in Patent No. 2,071,933. I have found that in a practical system, not only is such an arrangement not desirable, but it must be avoided at all costs lest, in the event of such an accidental setup, the detecting apparatus may be rendered overly sensitive. Since it may often be impractical, particularly in warehouses or the like, to shift the position of the contents thereof to conform to the peculiarities of the intruder detection apparatus, and since normal variations in the thermal condition of the room may cause the null point of the standing wave pattern to shift with respect to the microphone position at any time, it is essential that means be provided to eliminate this possibility.

To this end, I so relate the transmitting and receiving elements B and D as to ensure that reception never takes place at a null point of a standing wave pattern. This may be accomplished in varying ways, but insofar as the present invention is concerned, only one way, involving the employment of ka pair 'of transmitters,

each operating atya different frequency, need be disclosedl in order that the invention be properly understood.

I have discovered thatif two transmitters B and B are employed, each transmitting vibrations at a fixed frequencyof the same order of magnitude as the other but differing from one another by an amount exceeding the maximum detectable frequency, and preferably exceeding the maximux? detectable frequency by so great a degree as o be of a different order of magnitude therefrom, the effect of standing waves is substantially eliminated. The position of the null point of a standing wavefpattern is determined; by, among other things, the frequency thereof.v If, two appropriately related separate frequencies` are employed, the4 microphone. D4 can neverl befat the null point of both frequencies and consequently-the microphone D is. always in position to detect irrespective -of the positioning ofl the objects in the room A or of the normal thermal variations therein. I prefer that the frequencies of the two transmitters be between 18 and 20 kc. per` second and that they differ from oney another by an amount on the order of 1 kc. Aper second, 1 kc. per second being of a truly different order of magnitude from the maximum detectable frequency difference of the detectable frequency differences of between about 3 and 180 cycles per second.

'Ihis system and the apparatus employed to effectuate it is shown in schematic form in the block diagramof Fig. 2. Two transmitters B and B' are employed each of which is controlled 'by a separateoscillator It, lll' so that the waves C generated by the transmitter B will be sent out into the confined space A, there to impinge upon whatever air turbulence exists and to be reflected byA whatever objects are'contained therein as well as bythe walls thereof. The waves C leaving the refiecting object generally designated I2 are picked up by the microphone D which converts -them into corresponding electrical fluctuations, these` fluctuations being amplified in amplier lil and then sent outto a detector I6 where the frequencies of the received vibrations are compared in order to determine whether a frequency difference exists.v This may `be accomplished by combining all the received frequencies, if there be more. than one frequency,- and in therefore creating a beat frequency which will correspond to ,the difference between'thefrequencies. The thus detected frequency difference, if of sufficient magnitude and` amplitude, will actuate relay I8 so astoset off any desired sort of sensible alarm.

Circuit construction upon the change` oflength of a magnetostriction rod 2.0 vwhen subjected to varying magnetic field conditions. However, my invention is notr to be limited to this specific type of transmitter. The variation in length `of such magnetostriction rods is -.of-a, lvery small VorderT of magnitude suchas' a few 'parts` to a millionthA of the; lengthofthesrod.

However, if the rod betuneld to 'the-frequency of transmission, it will resonate'at that frequency and `thus give risey to vibrationsk of considerably greater amplitude. Consequently, each transmitter B must contain a magnetostriction `rod 2U tuned to the desired frequency of transmission. The magnetic fields of force active uponthetrod 2f! are provided by magnetostriction coils 2.2 and 24 which are here shown as connected toany suitable oscillator circuit generally designated '26. The frequency of oscillation of this vibrator circuit may be controlled byv meansof adjustable condenser 2t. The details of the oscillator Aliti and of the transmitterl may vary within wide limits as is well known to those skilledin the art, and their details forni'v no part of the present invention.

vThe circuit construction ofthe detector I6,`its connection with the relay I8, and its correlation with the various power sources therefore do, however, present many inventive features. The -sequence of operations in the detector areshown in a schematic way in Fig. 3. rlhe output from the amplifier, which consists'of electrical fluctuations of frequencies corresponding to the soundv waves received by the microphone D and thus having frequencies corresponding'to the transmitted waves C and the reflected waves C', isgfed to the tuned circuit t@ the output of 'which is still further amplified by amplifier 32 and then rectied by rectifier Si. Ifj the sound Waves received by the microphone D all have thesame frequency, the output of the rectifier 34 'will be constant. On the other hand, if' the microphone D receivessound vibrations ,of differing frequencies, those two frequencies when combinedv in amplifier Iii will give rise to an output having a varying amplitude the frequency of which will correspond to the frequency difference between the Vreceived vibrations. Hence, the output of the rectifier Cit will vary in accordance with the frequency difference received by the microphone D, both as regards amplitude and frequency.

Since a pair of transmittersB are employed, each transmitting at a different frequency from one another by, say l k. c. per second, there will normally be present in the output of the ampliner I@ a beat having a frequency equal to 1 k. c. per second. The output of rectifier 34 will fluctuate correspondingly. ff, because of the motion of an intruder inthe room A, the reflected waves C differ in frequency from the transmitted waves C, there will be present in thev output of the Y amplifier it a beat having a frequency corresponding to the speed of motion of the` intruder,

this beat having a'frequency between, forexample, 3 and 180 cyclesper second. The output of the rectifier will vary accordingly.

This output is then fed to a filter circuit 36 so designed as to reject all frequencies below the desired minimum detectable frequency as well as those high frequencies corresponding to the difference between the two transmitted frequencies. In addition, the filter circuit is so designed that the minimum passable frequency may be' controlled so that the sensitivity of the equipment to thermal disturbances and the like may be adjusted. Thus, the filter may be set to pass only frequencies above 3 cycles per second, in which case motion of an intruder at the rate of 2 inches per second will be detected but thermal disturbances other than those caused by a direct ame will not be detected. The yminimum passable free quen'cy might i lbees'et fat f-a lower value, fsay, 2v

7. cycles per second, in which case the apparatus will detect motion of an intruder at less than 2 inches per second, but would also detect thermal disturbances caused by, for example, a hot radiator. A minimum frequency of between 3 and 15 cycles per second has been found to be a safe value for an intruder alarm as distinguished from a fire alarm.

The output of the filter 36 is fed to amplifier 38 and is then rectified in rectifier 46. If the frequencies received by the microphone D are only those of the two transmitters B and B', the input to the rectifier 40 will be steady and consequently no output will result. This will be the situation when no intruder is present or when no excessive thermal disturbances obtain. However, if an intruder be moving, there will be an alternating current output from the filter 36 which can be rectified by the rectifier 4U and an output therefrom will result. This output is sent to an integrator 42 which permits the successive rectifications of the alternating current corresponding to the detected frequency difference to accumulate until their sum becomes strong enough to operate the relay i9 and thus set ofi an alarm of whatever nature is desired.

The rectifier 34, filter 36, and rectifier 46, together with the integrator 42 where employed, constitute the frequency detecting circuit. When the output of the microphone D is considered to consist of the original transmitted frequency and its modulation, then the rectifier 3i may be considered as performing a demodulating function while the rectifier 46 rectiiies the demodulated output of the rectifier 34. It will be noted from Fig. 3 that the output of rectifier 34 is also fed to relay I8. The purpose of this will become apparent upon consideration of the circuit diagram of Fig. 4. The microphone D which, for the frequencies under discussion is preferably of a crystal type, has its output connected to the grid of an amplifier tube 44 the output of which is reamplified by tube 46. As here illustrated, the two tubes are formed in a single envelope. The amplification accomplished is approximately 15 times per stage or a total of 225 times. The output from tube 46 goes to volume control 45 which is manually adjustable so as to determine the sensitivity of the apparatus to amplitude. It is this adjustment which permits the apparatus to be set up so as to be affected by a human but not, for example, by a rat or a mouse.

From the volume control 48 two more stages of amplification by tubes U and 52 are accomplished, the tubes 56 and 52 being here shown as contained in a single envelope similar to the tubes 44 and 46. The input of tube 52 is connected in a self-biasing arrangement designated generally by the numeral 54 so that a signal strong enough to drive the grid positive will produce, by grid rectification, enough charge on the condenser 56 to effectively neutralize the positive peak of the signal. Hence, when the signal iiuctuates in amplitude, as it will when different frequencies are received by the microphone D, the positive influence of the fluctuations will take place in a region where the grid of the tube is biased sufliciently negative due to the self-biasing arrangement 54 to make the plate of the tube respond to the fiuctuations in a manner such that the percentage modulation of the signal leaving this stage in amplification is increased because most of the signal, as it goes from its positive to its negative peak during the course of one cycle, will be beyond the negative cut-off point of the grid of the tube 52. This leaves but a small positive portion of the signal acting within the effective control range of the grid of the tube 52, and any fluctuations in amplitude of the total signal will be considerably magnified insofar as the output of the tube 52 is concerned due to the fact that those fluctuations will cover a much larger portion of the small positive signal which will control the grid of the tube 52 than of the total signal. The self-biasing circuit 62 described in the next paragraph operates in a similar manner.

This signal is fed to a tuned circuit 30, here shown as comprising a capacitance 60 and an inductance 58 connected in parallel so as to form a parallel resonant circuit, the tuned circuit 3D being tuned to approximately 19 k. c. so as to further amplify the modulated signal. A selfbiasing arrangement 62, similar to the arrangement 54, further increases the percentage modulation of the signal fed to the final amplifier stage 32 represented by tube 64.

The output of the amplifier stage 32 is fed to rectier 34 here shown in the form of diode 65 the output voltage of which follows the inuence of the signal and consequently generates a beat frequency representing the difference between the frequencies received by the microphone D. This output is fed to filter 36 which may be a network formed of capacitances and resistors, the resistors 66 being adjustable so as to control the pass characteristics of the filter. It is by means of this adjustment that the minimum and maximum detectable frequencies of the apparatus may be varied, thus varying the minimum and maximum detectable speeds of motion and the minimum and maximum detectable thermal or air disturbances. The filter 36 will not pass frequencies on the order of 1 K. C. and consequently the difference in frequency between the transmitters B and B is blocked. The filter circuit 36 will however pass frequencies in a band having a lower limit of perhaps between 3 to 15 cycles depending on the undesired turbulence conditions, and an upper limit of cycles per second and consequently those frequencies, corresponding to motion of an intruder or undue thermal or air disturbance, will pass to the amplifier stage 38 represented by tube 68 the output of which is fed to the rectifier stage 40 represented by tube 10. As here illustrated, the tubes 68 and 16 aswell as the tubes 64 and 65 are enclosed in a single envelope.

If the only detected frequency difference is that between the two transmitters B and B', no signal will pass the filter 36. There will be no input to the amplifier 38, and there will consequently be no output from the rectifier 40. If however, there is a frequency difference detected between, say, 3 and 180 cycles per second, an alternating current of that frequency will be passed by the lter 36, amplified by the amplifier 38 and rectified by the rectifier 40 so that an output will be obtained from the latter. As here illustrated, only half wave rectification is obtained and the rectified impulses are fed to an integrator 42 here shown as a condenser 'I2 which serves to accumulate the rectified impulses and build up a charge which is then fed to relay I8 so as to operate the same.

It therefore will be appreciated that the frequency detecting circuit of the present invention will have two outputs, one from the rectifier 34 which is dependent upon the existence of a given frequency and one from rectifier 40 which is dependent upon the existence of a different frequencmthe filter-circuit 36 serving to separate the two reciers 34 and i0 and to permit only thel second frequency, when present, to pass to the -rectiiier 40.

Control circuit My control circuit and its manner of connection present many inventive features, particularly when employed in conjunction with the frequency detecting circuit just described. It comprises a relay |.S which is composed of two opposed windings T4 and 55, a current through each being independently controlled by a separate tube i8 and 80 which for convenience may be combined into a single envelope. The output of the integrator 42 is connected to the grid S2 of tube '53. The gridte of tube 80 is connected by means of electrical connection 3c (see Figs. 3 and 4) to the condenser 8S which is in turn connected to the output of the rectier 34. The condenser 88 acts as an integrator in the same manner as the condenser i2.

Whenever there is an output from the rectifier 34, which will be Whenever both transmitters Band B' are operating and the tubes 44, 46, 50, 52, 64 and 65 are all functioning properly, the grid 84 of tube 30 will be sobiased that no current will flow through the tube S and consequently no current will flow through the winding l5. Should any portion of the system before and including the condenser 88 fail, this negative bias will be removed from the grid 8d and current will flow through the tube 80 and the winding 16. The Winding l5 is so wound in conjunction with the armature of the relay i8` that current iiowing therethrough'will cause the relay to trip and thus set'oif whatever alarm is desired. Consequently, upon failure ofthe apparatus, the alarm will be setol `The tube '|8 normally passes current, which current iiows through the winding i4, thus 'tending to hold the relay out of its tripping position. The output of the rectifier 40 as integrated by the condenser l2, which output is present when a frequency difference corresponding to motion of an intruder is detected, puts a negative bias on the grid [i2 of tube lt, thus cutting off current ow to the winding i4 and thus permitting the relay to trip and set oii` the alarm.

It will therefore be'. apparent thatduring normal operation of the apparatus, when no intruder is detected, current flows through-winding 14 and no current iiows through winding 16, the winding ifi being wound in a sense such as 'to keep the relay from tripping. When an lintruderis detected, the current through winding 'M is cut off and the relay trips, setting off a sensible alarm. Should the apparatus fail (by apparatus is included the oscillator l0 and the transmitter B as well the other elements thereof), there will be no bias on either of the 'grids 82 or 84 and consequently current Will flow through both of the coils 'i4 and is but, since they are wound in opposed senses, they Vwill neutralize one another and the relay will .design can fully understand the circuits shown .iI1-Figs.-4, 5 and 6 and can compute proper circuit-values to achieve the described results, for

purposes'of clarity certain of the more signifi-.- cant `circuit values .willbe now set forth. Tubes 44, 46and 50,52 maybe of thetype 3A5. Tubes 04, 65 and 68, 10 may be-of the type |S5. Tube 18, may bey of the type 3A5. In the self-- biasing circuit 54, the resistor 55 may have a value `of 2 megohms and the capacitor 56 may have a value` of- .l microfarad. In the selfbiasing circuit 62 the resistor 6| may have a value of 2 megohms and the capacitor 60 may havea valueof .1 microfarad. In the resonantly tuned circuit 30 .each of the primary and sec- )ndary of the transformer 58 mayvhaveaninductance of y 30 millihenries, the coupling-coefficient betweenthe coils-being closeto unity, While the capacitor 60.1nay `be Variable between .0005 inicrofarad and .0015.microfarad. The integrating condenser |2` may have a Value of.5.micro farad and the integrating condenser 881 may have a value of .01 microfarad. In the filter circuit 36, the resistors 66 may be variablefbetween .25 and5 megohms, the capacitor GTmay have a value of .04 microfarad andthe capacitors 69 may leach have a value of .0l microfarad.

Power supply The powerv supply for `the apparatus is shown inFig. 6 andV is vthere illustrated as, being adapted for use with aprimary` power source of alternating current and a secondary power -sourceof direct current 92, the latter comprising a B- battery Sllvof .approximately 135 volts and an A-batteryv 96 ofapproximately 12v volts. The alternating currentsource 90 -may comprise `a =volt source, such as is commonlyl found `in industrial and rresidential locations, which leads to fusesjg andV tocontact points |00 and |02, these points'beingconnectable to points |06 Vand |08. An on-off switch ||0is provided so that the apparatus may be turned offwhile the room A is `occupied but may be turned on when it is desired 'that the apparatus operate to detect an intruder. the alternating current line and is provided with anarmature 'H2 which controls contact making blades (not shown). Selenium rectiers H6 are interposed between the coil `I I8 ofthe relay m4 andthe alternating current source 90 so that uni-'directional Vcurrent passes through the coil I8 in an amount determined by the magnitude of the alternating current or primary power source 90. So long as that magnitude exceeds a certain critical amount,v the contact blades will take up a position to cause the apparatus to .we energized by the alternating current source .1%. This source is connected to the apparatus via selenium rectiers |20 each having a p rotective resistor |22, the rectiiers charging electrolytic condensers |24 which, through the relay contacts |26. and |28, charge another Velectrolytic condenser |30. A filter system generally. designated |32 follows to. smoothv ripples in the .rectifier output. From here thepcwer supply is` divided, into two sections, one comprising a choke coil |34- in series with a. voltage regulator tube |3.6which supplies voltageY at a point designated |33 for the oscillatorZB. AThe other section consists of a resistor |40 and a voltage regulator tube |42 which supplies voltage for the input audio amplifier stagesV at point |44. The -resistor |40 yis tappedby means of movable connector |'4S which supplies voltage to point |48 and thence to the tubes ,18.and80 viaitherelay windings 'I4 and 1B;

The'relay |04 is connected across Also leading from the condensers |24 across the input is a circuit comprising contacts |58 and |52, in series with which are the filaments |54 and |54 for the tubes of the oscillator 26. From here the electrical circuit goes through contact points |58 and |60 and choke coil |62 to another filter condenser |64 and through resistor |66 to the filamentsr |68 and |10 of tubes 44, 46, D and 52, each of these filaments being protected by means of by-pass condensers y |12. Point |14 is connected to the corresponding points |14 in Fig. 4. The filament |16 for the relay tubes 18 andll is next in line and also provided with a by-pass condenser |18 and point |80 is connected to the corresponding points |60 of Fig. 4. The filaments |82 and |84 for tubesA 64 and 65 and tubes B8 and 'l0 respectively are next in line, these being provided with 'oy-pass resistor |86. The resistor |66 is provided with `movable tap |88 which leads to contact point The direct current supply 92 is provided with an individual on-off switch |92 which is normally closed and the high potential sides of the B and A batteries 94 and 96 lead to contact points |94 and |96.

The frequency detecting circuit above described and the control circuit with which it is employed provide a means of actuating any external control device in accordance with the presence or absence of a plurality of frequencies. As here illustrated, the control device is actuated only when both frequencies are present or when neither frenuencv is present. This h-as the advantage that it will cause the alarm to be actuated whenever the power lsupply or any portion of the circuit should fail. It is, of course, equally possible to employ the frequency detecting circuit and control circuit of the present invention so as to actu- Iate the external control device when 4only one of the frequencies is present, but not when both are present. if that should be deemed desirable in a given application.V It will also be apparent that many variations may be made in the mode of Iachieving rectification land integration, land that integration need not be employed if the other circuit conditions so permit. Manv other changes may be made in the details of the disclosure of this application with-out departing from the spirit of the invention as defined in the following claims.

Icl-aim:

' 1. Electrical apparatus for detecting the presence or absence of two freduencies of electrical fiuctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for receiving said original signal `at one end thereof, a first rectifier connected in said principal circuit after said one end for rectifying said fiuctuations and a second rectifier connected in said principal lcircuit after said first rectifier for rectifying the electrical output of said first rectifier if said output is non-constant, a relay switch having first and second windings effective when energized to oppose one another, first and second tubes ea-ch having a control electrode and an output electrode, the output electrode of each tube being connected to one of said windings so that the current through each tube will energize the respective windings, and a pair of integrating circuits each connected between the control electrodes of said first and second tubes respectively and a point in said principal circuit after said first and second rectifiers respectively, said tubes being biased in such a way that the current through said tubes Will be altered l2 when the rectifiers connected thereto rectify fluc tuating currents.

2. Electrical apparatus for detecting the Ipresence or absence of two frequencies of electrical fluctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for receiving said origina-l signal at one end thereof, a first rectifier connected 1in said principal circuit after said one end for rectifying said fluctuations and a second rectifier connected in said principal circuit after said first rectifier for rectifying the electrical output of said first rectifier if said output is non-constant, a relay switch having first and second windings effective when energized to oppose one another, first and second tubes each having a control electrode and an output electrode, the output electrode of each tube being connected to one of said windings so that the current through each tube will energize the respective windings, and a pair of integrating circuits each connected between the control electrodes of said first and second tubes respectively and a point in said principal circuit after said first and second rectifiers re-f spectively, said tubes being biased in such a way that the current through said tubes Will be de creased when the rectifiers connected thereto rec-f tify fluctuating currents.

3. Electrical apparatus for detecting the presi ence or absence of two frequencies of electricalfluctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for receiving said original signal at one end thereof, a first rectifier connected in said principal circuit after said one end for rectifying said fluctuations and a second rectifier connected in said principal -circuit after said first rectifier for rectifying the electrical output of said first rectifier if said output is non-constant, a relay switch having first and second windings effective when energized to oppose one another, first and second tubes each having a control electrode and an output electrode, the output electrode of each tube being connected to one of said windings so that the current through each tube will 'energize the respective windings, and a pair of integrating circuits each connected between the control electrodes of said first and second tubes respectively and a point in said principal circuit after said first and second rectifiers respectively, said tubes being biased in such a way that the current through said tubes will be cut ofi when the rectifiers connected thereto rectify fluctuating currents.

4. Electrical apparatus for detecting the pres'- ence or absence of two frequencies of electrical fluctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for reciving said original signal at one end thereof, a first rectifier connected in said principal circuit after said one end for rectifying said fiuctuations, a filter circuit connected in said principal circuit after said first rectifier and effective to pass only fluctuations of frequencies within a predetermined range, and a second rectifier connected in said principal circuit after said filter circuit for rectifying the filtered electrical output of said first rectifier if said output is non-constant and has a frequency capable of being passed by said filter circuit, a relay switch having first and second windings effective when energized Yto oppose one another, first and second tubes each having a control electrode and an output electrode, the output electrode of each tube being connected to one of said anzeiger y'windingsf so.; :thattlre- `currentf through.- eachl tube '.willcenergize': the: respectivefwindings, yan'dfia pair of .integrating circuits :Leach connected between the control electrodes of said first and second Atubes; respectively and a, point in said principal `circiiityaztter said first' and` secondrectifiers recircuit will pass frequencies only in a range having a minimum between 3 and 1.5 cycles per second and a maximum .on .the order yof magnitude oi 180 cycles per' second.

6. .'The apparatus of claim-5, in which said filter 'circuit' is adjustable so. .that the minimumof -the passedfrequency range maybe varied.

"I, .Thezapparatusof claim 4', i-n whichsaid filter "circuit will .passfrequencies in Ja rangeihaving a minimumbetween and' lxcycles per second.

,Thefapparatusof claim 7in'which said filter circuit is adjustable so that the minimum of the passed frequency range may be varied.

9. The apparatus of claim 4, in which said filter circuit is adjustable so that the range of frequencies which it will pass may be varied.

10. Electrical apparatus forrdetecting the presence or absence of two frequencies of electrical fluctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for receiving said original signal at one end thereof, a rst rectifier connected in said principal circuit after said one end for rectifying said fluctuations, a filter circuit connected in said principal circuit after said rst rectifier and effective to pass only fluctuations of frequencies within a predetermined range, and a second rectifier connected in said principal circuit after said filter circuit for rectifying the filtered electrical output of said first rectier if said output is non-constant and has a frequency capable of being passed by said filter circuit, a relay switch having first and second windings effective when energized to oppose one another, first and second tubes each having a control electrode and an out put electrode, the output electrode of each tube being connected to one of said windings so that the current through each tube will energize the respective windings, and a pair of integrating circuits each connected between the control electrodes of said first and second tubes respectively and a point in said principal circuit after said first and second rectifiers respectively, said tubes being biased in such a way that the current through said tubes will be decreased when the rectifiers connected thereto rectify fluctuating currents.

11. Electrical apparatus for detecting the presence or absence of two frequencies of electrical fluctuation in an original signal and actuating a switch means in response thereto comprising a principal circuit for receiving said original signal at one end thereof, a first rectifier connected in said principal circuit after said one end for rectifying said fiuctuations, a filter circuit connected in said principal circuit after said first rectifier and eective to pass only fluctuations of frequencies within a predetermined range, and a second rectifier connected in said principal circuit after said filter circuit for rectifying the filtered electrical output of said first rectifier if said output is non-constant and has a frequency capable of being passed by said filter circuit, a relay switch having rst and second windings effective when energizedv to; opposeone` another; irst andi *second tubesk each having a'control electroderand anoutput electrode, the,v outputelectrode.- ofeach tube being Aconnected tor one of said windingsso that the current through each tube' will energize the respective` windings, and a pair ofintegrating-circuits each .connected between the* controlfelectrodes. of` said* firstl and second tubes, respectively andiapont in said principal circuit after saidiirst andsecond rectifiers respectively, saidtubes being biasedinsuch a way that. the currentthrough said tubes will be cut off when the rectiiiers connected thereto rectify fluctuating.currents.V

.12. Apparatus. for controlling"v a.. switch: means in responseto the.. presence or.I absencemf; one: or more. frequencies of electric al1 fluctuations: in: an original signal, thefrequencies diiferingfv-by-an amount within a ,c predetermined: range, .com-

' prisin-g ia .principal -ic'ircuit having anv input; portion at one end for receiving said originalsgnal, a rst rectifier connected acnoss said input portion, la second rectifier :connected across 1 said input portion after said first rectifier' and electrically coupled to the output of said first rectifier, a filter circuit between said rectifiers effective to filter out all frequencies other than those lof the desired frequency difference range, a switch having an actuating circuit sensitive to the presence or absence of a pair of switch signals, and independent direct electrical connections between said switch actuating circuit and the outputs of said first land second rectifiers, said first rectifier having an output defining a first switch signal when said original signal has at least one frequency of fluctuation, said seclond rectifier having an output defining a second switch signal, derived from re-rectifying la portion of the output of said rst rectifier, when said original signal has more than one frequency of fluctuation.

13. The apparatus of claim 12, in which said filter circuit will pass frequencies only in a range having a minimum between 3 and 15 cycles per second and a maximum on the order of magniitude of 'cycles per second.

14. The apparatus of claim 13, in which said filter circuit is adjustable so that the minimum of the passed frequency range may be varied.

15. The apparatus of claim 12, in which said switch actuating circuit is effective toi actuate said switch in one sense when it receives neither or both of said switch signals and in another sense when it receives one of said switch signals alone.

16. The apparatus of claim 15, in which said filter circuit is adjustable so that the minimum of the passed frequency range may be varied.

17. Apparatus for controlling a switch means in response to the presence or absence of 'a modulation of lone or more frequencies impressed upon an original signal which comprises a principal Icircuit having an input portion at one end for receiving said original signal, a switch having an actuating circuit sensitive to` the presence or `absence of a pair of switch signals, means in said principal circuit for demlodulating said original signal, means electrically connecting a portion of fthe output of said demodulating means directly to said switch actuating circuit, a rectifying means, means electrically connecting la portion of the output 'of said demodulating means to said rectifying means, said last named connecting means including a, filter effective to filter out all frequencies other than those of the modulation, and means for electrically connecting a. portion of the output of said demodulating means to said rectifying means, said last named connecting means including a filter effective to lter out all frequencies other than those of the modulation, and means for electrically connecting the output of said rectifying means to said switch actuating circuit, said demodulating means having an output defining a first switch signal so long as said original signal is present, said rectifier having an output defining a second switch signal when said original signal has a modulation.

18. The apparatus of claim 17, in which said filter circuit will pass frequencies only in a range having a minimum between 3 and 15 cycles per second land a maximum on the order of magnitude of 180 cycles per second.

19. The apparatus of claim 18, in which said filter circuit is adjustable so that the minimum of the passed frequency range may be varied.

20. The apparatus of claim 17, in which said switch actu-ating circuit is effective to actuate SAMUEL M. BAGNO.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,490,958 Brown Apr. 22, 1924 1,669,524 Kintner May 15, 1928 2,167,007 Schott July 25, 1939 2,382,847 Baumann Aug. 14, 1945 2,422,064 Anderson June 10, 1947 2,455,639 Anderson Dec. 7, 1948 

